This study quantified soil biogeochemical processes, and plant biodiversity (species numbers, abundance, species evenness and functional diversity), and examined the interrelationships in natural (terrestrial), invaded and cleared riparian ecotones in the Western Cape.
The results of this study support the hypothesis, that invasion by acacia species has an impact on soil and microbial processes, but that these processes can recover when invaders are cleared. The results find key application in the Working for Water Program, in that removal of Acacia species may assist in the restoration of riparian ecotones. Although plant structure and diversity was not studied in depth, those aspects of function that received attention, indicate recovery of the natural state after the removal of Acacia species. However, more than seven years after the removal of Acacia species, effects (such as high available N) still remain, which need to be carefully considered in managing any clearing and follow-up activities.
Seasonal measurements of selected parameters were collected in reference (mainly un-invaded) and cleared riparian site. The results point to changes in soil properties, soils processes and soil microbial diversity where invasive Acacia spp. establish in large numbers in fynbos riparian ecotones. Invasion affects soil processes by increased nitrogen availability, soil carbon efflux and soil phosphatase activity. As a result microbial diversity also changes in terms of diversity and structure.
Understanding of the magnitude of soil nitrogen mineralization, de-nitrification and soil respiration and feedbacks with soil and plant biodiversity provided valuable insights into the consequences of alien invasive species for riparian functioning, and also suggested new knowledge for rehabilitation strategies in cleared riparian areas. When the Acacia species are removed, soil properties, soil processes and soil microbial diversity recover, but the process requires time.